The practice of adopting a variable-speed operation of a rotating electric machine by use of an inverter is growing steadily from an energy-saving point of view in recent years. However, it has been reported (refer to “Technical Report of Institute of Electrical Engineers of Japan”, No. 1218, pp. 60-69) that various problems occur with respect to insulation of the rotating electric machine in the case of driving the rotating electric machine by use of the inverter. For example, it has been reported that if an inverter surge voltage generated due to ON/OFF of a switching element inside an inverter propagates through a cable to reach an end of the rotating electric machine, this will cause the inverter surge voltage to jump at the end of the rotating electric machine caused by mismatching in serge impedance between the cable and the rotating electric machine, whereupon a voltage higher than a voltage at the time of conventional commercial frequency sine wave driving is applied to the rotating electric machine, thereby causing insulation degradation.
Further, it has been reported that upon intrusion of a steep inverter surge voltage into a rotating electric machine, a high voltage is generated between turns of a coil, on the lead side of a winding of the rotating electric machine, thereby causing insulation degradation. Further, it has also been reported that if a slot-corona preventive layer, and an electric field relaxation layer are provided on the surface of a ground insulator, there is a possibility that heat generation, and degradation occur to these layers.
Further, since a switching frequency of the inverter is in a range of several hundred Hz to 10 kHz, there is a possibility that these phenomena repeatedly occur with high frequency as compared with the case of a conventional commercial frequency sine wave driving. For such reasons described as above, with a rotating electric machine for use in inverter-driving, it is necessary to design these insulation parts so as to enable them to withstand the inverter surge voltage, and to test a rotating electric machine already completed in order to check whether or not those insulation parts can withstand a predetermined inverter surge voltage. Furthermore, it is necessary to test the inverter drive rotating electric machine even after delivery thereof in order to check whether or not abnormal insulation degradation has occurred due to application of an inverter operation voltage, thereby running a diagnosis on insulation degradation.
With a commercial frequency sine wave drive rotating electric machine, an insulation test on the ground insulation of a winding has thus far been conducted in the past, the insulation test including, for example, an AC current test, a dielectric loss tangent (tan δ) test, and a partial discharge test (refer to “Modern High-Voltage Engineering”, by Masayuki Yada, Published by Ohm Co., Ltd., pp. 186-192, “Electrical Joint Study”, compiled by Electrical Joint Study Society, Vol. 51, No. 2, pp. 59-60, 1995, and “Electrical Equipment Test Method Outline”, complied by Institute of Electrical Engineers of Japan”, Technical Report of Institute of Electrical Engineers of Japan”, No. II, No. 18, pp. 38-49, 1972). More specifically, if the rated voltage of a rotating electric machine is E, a voltage E/√{square root over (3)} is applied against the ground (between cores) of a winding of the rotating electric machine, so that, in these tests, the voltage E/√{square root over (3)} as a test voltage has been applied to the ground insulation, thereby having measured the AC current, the tan δ, and the partial discharge, respectively. Further, if there exists stipulation with respect to specification, in particular, and so forth, a sine wave voltage equivalent in magnitude to the rated voltage E has been applied to the ground insulation, having thereby measured the AC current, tan δ, and the partial discharge, respectively. A measurement value from each of these tests has been evaluated in the light of a pass/fail criterion value established from the past insulation tests conducted on the commercial frequency sine wave drive rotating electric machine, thereby having determined pass/fail on an insulation state.